Carbon fibers are lightweight and excellent in strength and elastic modulus, and thus are combined with various matrix resins to form a composite material, which is used in various fields such as aircraft members, spacecraft members, automobile members, ship members, constructional materials, and sporting goods. In order to impart the excellent characteristics of carbon fibers to a composite material including the carbon fibers, excellent adhesion between the carbon fibers and a matrix resin is important.
In order to improve the adhesion between carbon fibers and a matrix resin, the carbon fibers are typically subjected to oxidation treatment such as gas phase oxidation and liquid phase oxidation, and thus an oxygen-containing functional group is introduced to the surface of the carbon fibers. For example, a disclosed method includes subjecting carbon fibers to electrolysis to improve interlaminar shear strength as an index of the adhesion (see Patent Literature 1). However, as the level of characteristics required for a composite material has increased in recent years, the adhesion achieved by such an oxidation treatment alone is becoming insufficient.
Carbon fibers are brittle and poor in bindability and abrasion resistance and thus readily generate fluffs or broken thread in a high-order processing step. To address this problem, methods of coating carbon fibers are disclosed (see Patent Literatures 2 and 3).
For example, a disclosed method includes applying, as a sizing agent, a diglycidyl ether of bisphenol A onto carbon fibers (see Patent Literatures 2 and 3). Another disclosed method includes applying, as a sizing agent, a polyalkylene oxide adduct of bisphenol A onto carbon fibers (see Patent Literatures 4 and 5). Another disclosed method includes applying, onto carbon fibers, a sizing agent that is obtained by adding an epoxy group to a polyalkylene oxide adduct of bisphenol A (see Patent Literatures 6 and 7). Another disclosed method includes applying, as a sizing agent, an epoxy adduct of polyalkylene glycol onto carbon fibers (see Patent Literatures 8, 9 and 10).
In addition, another disclosed method includes applying, as a sizing agent, a urethane compound having an epoxy group and a quaternary ammonium salt onto carbon fibers (see Patent Literature 11). The disclosed method also improves the bindability and the abrasion resistance but has failed to improve the adhesion between carbon fibers and a matrix resin.
It is known that these methods improve the bindability and the abrasion resistance of carbon fibers.
However, these previously disclosed methods have no technical idea to positively improve the adhesion between carbon fibers and a matrix resin by means of the sizing agent and cannot greatly improve the adhesion between carbon fibers and a matrix resin in practice.
In order to improve impregnation properties of a matrix resin into carbon fibers, a method of coating carbon fibers with a particular sizing agent has been carried out.
For example, a disclosed method includes applying, as a sizing agent, a cationic surfactant having a surface tension of 40 mN/m or less and a viscosity at 80° C. of 200 mPa·s or less onto carbon fibers (see Patent Literature 12). Another disclosed method includes applying, as a sizing agent, an epoxy resin, a water-soluble polyurethane resin, and a polyether resin onto carbon fibers (see Patent Literature 13). These methods are proved to improve the bindability of carbon fibers and the impregnation properties of a matrix resin into carbon fibers. However, these previously disclosed methods also have no technical idea to positively improve the adhesion between carbon fibers and a matrix resin by means of a sizing agent and cannot greatly improve the adhesion between carbon fibers and a matrix resin in practice.
As described above, a sizing agent has been used as what is called an adhesive for improving high-order processability or for improving the impregnation properties of a matrix resin into carbon fibers, and little study has been made on the sizing agent to improve the adhesion between carbon fibers and a matrix resin. Even when studied, sizing agents achieve limited effects. In other words, the adhesion is insufficiently improved, or the effect is achieved only in the case of a combination with particular carbon fibers.
For example, a disclosed method includes applying, as a sizing agent, N,N,N′,N′-tetraglycidyl metaxylylene diamine to carbon fibers (see Patent Literature 14). The disclosed method provides improvement in interlaminar shear strength as an index of the adhesion when compared with the case using glycidyl ether of bisphenol A, but the improvement effect of the adhesion is still insufficient. In addition, the N,N,N′,N′-tetraglycidyl metaxylylene diamine used in the disclosed method contains a tertiary aliphatic amine in its structure and thus exhibits nucleophilicity, which unfortunately causes the diamine to undergo self-polymerization. This hardens carbon fiber bundles with time to deteriorate high-order processability.
Another disclosed method includes applying, as a sizing agent, a mixture of a vinyl compound monomer having a glycidyl group and an amine curing agent for an epoxy resin onto carbon fibers (see Patent Literature 15). The disclosed method provides improvement in interlaminar shear strength as an index of the adhesion when compared with the case without the amine curing agent, but the improvement effect of the adhesion is still insufficient. In addition, the glycidyl group reacts with the amine curing agent to increase the molecular weight during a process of drying the sizing agent, and this hardens carbon fiber bundles to deteriorate high-order processability. Moreover, the space among carbon fibers becomes small, and this reduces the impregnation properties of a resin.
Another method is also disclosed as the method using a sizing agent containing an epoxy compound and an amine curing agent in combination (see Patent Literature 16). The disclosed method improves the handleability and the impregnation properties of fiber bundles, but the sizing agent is polymerized to form a film on the surface of carbon fibers. This may interfere with the adhesion between the carbon fibers and an epoxy matrix resin.
Another disclosed method includes applying an amine compound onto carbon fibers (see Patent Literature 17). The disclosed method provides improvement in interlaminar shear strength as an index of the adhesion when compared with the case without coating, but the improvement effect of the adhesion is still insufficient. The disclosed method describes no detailed mechanism of improving the adhesion, but the mechanism is supposed as below. In the disclosed method, the amine compound used is diethylenetriamine or xylenediamine containing a primary amino group, or piperidine or imidazole containing a secondary amino group. Each amine compound contains an active hydrogen in the molecule, and the active hydrogen can cause an epoxy matrix resin to accelerate the curing reaction. For example, an epoxy matrix reacts with the amine compound to form a hydroxy group. The hydroxy group interacts with a carboxy group, a hydroxy group, and other groups on the surface of carbon fibers to form hydrogen-bonds, and this improves the adhesion. As described above, the disclosed method unfortunately still provides insufficient improvement result of the adhesion, which does not satisfy the requirements for recent composite materials.
Another disclosed example of using an amine compound as the sizing agent is a method using a cured product of a thermosetting resin and an amine compound (see Patent Literature 18). The disclosed method uses m-xylenediamine containing a primary amino group and piperazine containing a secondary amino group as the amine compounds. An object of the disclosed method is to improve the bindability and the handleability of carbon fiber bundles by actively reacting an active hydrogen contained in the amine compound with a thermosetting resin typified by an epoxy resin to form a cured product. The carbon fiber bundles are limited to be used for a chopped application and still have insufficient mechanical characteristics relating to the adhesion of a formed piece after melting and kneading with a thermoplastic resin.
Still another disclosed method includes using carbon fibers having a surface oxygen concentration O/C within a particular range, a surface concentration of hydroxy groups within a particular range, and a surface concentration of carboxylic groups within a particular range and applying, as a sizing agent, an aliphatic compound having a plurality of epoxy groups onto carbon fibers (see Patent Literature 19). Although the disclosed method provides improvement in EDS as an index of the adhesion, the method still achieves insufficient improvement effect of the adhesion between carbon fibers and a matrix resin, and the improvement effect of the adhesion is limited and is achieved only in the case of a combination with particular carbon fibers.